1. Field of the Invention
The present invention relates to a laminated structure in which insulator layers and electrode layers are alternately stacked, and a method of manufacturing the laminated structure. Further, the present invention relates to an ultrasonic transducer array including a plurality of such laminated structures, to be used for ultrasonic diagnosis, nondestructive inspection and so on.
2. Description of a Related Art
Laminated structures in each of which insulator (dielectric) layers and electrode layers are alternately formed, are utilized, not only in laminated capacitors, but also in various other uses such as piezoelectric pumps, piezoelectric actuators and ultrasonic transducers. In recent years, with the developments of devices and equipment concerning MEMS (micro electromechanical systems), elements each having such a laminated structure have been microfabricated still further and packaged more highly.
In the microfabrication of an element having opposed electrodes, the smaller the area of the element is made, the smaller the capacitance between the electrodes becomes, so that a problem of rise in the electric impedance of the element occurs. On this account, when the electric impedance rises in a piezoelectric actuator, for example, the impedance matching between the piezoelectric actuator and a signal circuit for driving the piezoelectric actuator cannot be established, and it becomes difficult to feed power to the piezoelectric actuator, and thereby, the performance of the piezoelectric actuator is degraded. On the other hand, in an ultrasonic transducer employing a piezoelectric element, oscillation intensity of ultrasonic wave is dropped. Accordingly, in order to enlarge the capacitance between electrodes while microfabricating the element, it has been practiced to alternately stack a plurality of piezoelectric material layers and a plurality of electrode layers. This is because the capacitance between electrodes of the entire element can be enlarged by connecting the plurality of stacked layers in parallel.
In such a laminated structure, in order to connect the plurality of electrode layers to one another, interconnections are formed from side surfaces of the laminated structure. FIG. 9 is a sectional view for explanation of a general interconnection method of a laminated structure. A laminated structure 100 includes a plurality of piezoelectric material layers 101, a plurality of layers of electrodes 102 and 103 and side electrodes 104 and 105. The electrode 102 is so formed that one end thereof extends to one wall surface of the laminated structure, and the electrode 102 is connected to the side electrode 104 and insulated from the side electrode 105. Further, the electrode 103 is so formed that one end thereof extends to the other wall surface of the laminated structure, and the electrode 103 is connected to the side electrode 105 and insulated from the side electrode 104. By applying a potential difference between the side electrode 104 and the side electrode 105, a voltage is applied to the piezoelectric material layers 101 disposed between the electrodes 102 and 103, and the piezoelectric material layers 101 expand and contract by the piezoelectric effect.
By the way, as shown in FIG. 9, in the electrodes 102 and 103, insulating regions 106 where no electrode is formed are provided for insulating the electrodes from one of the side electrodes. The insulating regions 106 do not expand or contract even when a voltage is applied to the laminated structure 100. On this account, a problem that stress is concentrated on this part and the part is easy to break occurs.
As another interconnection method in the laminated structure, Japanese Patent Application Publication JP-P2002-118305A discloses an interconnection method of a multi-electrode piezoelectric device having a piezoelectric or an electrostrictive material with a large number of independently controlled electrodes, wherein a part or an entire of an electric circuit board having the piezoelectric/electrostrictive material on which electrodes for external connection are formed on the surface is coated with an insulative material, an interconnection pattern is formed on the surface thereof by removing the insulative material applied onto the electrodes for external connection, and a desired conduction between the electrodes and the interconnection pattern is established. However, forming interconnection to each of the large number of laminated structures in accordance with such method is complicated, and especially, in the case where the laminated structure is arrayed in a two-dimensional manner, the interconnection is difficult to be formed.